Recently, many brushless motors have been used to drive outdoor fans to be installed in the outdoor unit of air conditioners. Even if not driven by motor, the outdoor fan may be rotating by wind, at different rotational directions and speeds. When an outdoor fan happens to be rotating, first, the fan should be applied with brake, stopped and set to home position to start the motor, before the operation of the air conditioner is started.
Normally, the brushless motor necessitates a sensor to detect the present rotor position, that is, the position of the magnetic pole. When braking to stop and setting the outdoor fans to home position, actual rotational direction and speed of the fan are identified based on the output of the sensor, and current is supplied into the stator coil according to the detection data. With the driving systems according to the prior art, however, they used one unit of Hall probe for each phase, that is, three in total, as position sensor, thus causing a complicated control, resulting in high cost.
On the other hand, with the driving systems according to the prior art, when braking to stop and setting to home position, DC current is supplied into stator coil, thus resulting in a possibility of demagnetizing the permanent magnet forming the rotor, or of an excessive current being supplied to destroy the driving circuit for energizing stator coil.